Why is RAX Pushed onto the Stack First in 64-bit Function Calls?

Why is RAX Pushed to the Stack as the First Operation?

In the assembly of the C code provided, RAX is pushed to the stack as the first operation to maintain proper stack alignment according to the 64-bit ABI.

The ABI (Application Binary Interface) specifies a set of rules for calling conventions between different parts of a program. In the 64-bit ABI, the stack must be aligned at 16-byte boundaries before a call instruction is executed.

The call instruction pushes an 8-byte return address onto the stack, breaking the 16-byte alignment. To correct this, the compiler needs to perform an alignment operation.

Pushing a don't-care value (such as RAX) achieves this alignment and can be more efficient than alternative methods like subtracting 8 bytes from the stack pointer. On CPUs with a stack engine, pushing RAX can take advantage of the hardware's ability to efficiently handle stack operations.

In summary, the push rax instruction is used to maintain the stack alignment required by the 64-bit ABI, ensuring that subsequent function calls operate correctly from a properly aligned stack.

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